Recently, weight saving of tires has been pursued because of strong social demands for fuel efficiency of automobiles. Of tire members, weight saving has also been required for an air shutoff layer (inner liner) which is disposed inside a tire and is required to reduce leakage of air from the inside to the outside of a pneumatic tire.
At present, for a rubber composition for the air shutoff layer, a rubber formulation mainly composed of butyl rubber which contains, for example, 70 to 100% by mass of butyl rubber and 30 to 0% by mass of natural rubber is used to improve air permeation resistance of the tire. Further, the rubber formulation mainly composed of butyl rubber contains, in addition to butylene, about 1% by mass of isoprene, which allows intermolecular co-crosslinking with an adjacent rubber layer along with sulfur, a vulcanization accelerator, and zinc white. The butyl-based rubber having a general formulation is required to have a thickness of about 0.6 to 1.0 mm for tires for passenger cars, and a thickness of about 1.0 to 2.0 mm for tires for trucks and buses. To pursue weight saving of tires, there has been a demand for a polymer which is more excellent in air permeation resistance and allows a further reduction in the thickness of the air shutoff layer, when compared with the butyl-based rubber.
In molding a green tire for a pneumatic tire, when an inner liner P is molded on a drum 5A as shown in FIG. 7, generally, an inner liner film P2 is bonded to an unvulcanized inner liner rubber P1 beforehand on a conveyer, with positions of both end edges in a longitudinal direction being aligned, to produce a laminate, the laminate is wound on a band over its entire circumference with inner liner film P2 of the laminate being disposed on an inner surface side, both end portions of the laminate are overlapped with each other at one location on the circumference to form a joint PJ, and thereafter a stitching roller is used to press joint PJ of the laminate and remove air.
In such a technique, since inner liner film P2 and unvulcanized inner liner rubber P1 are bonded beforehand with the positions of their both end edges in the longitudinal direction being aligned, and thereafter wound on the drum, joint PJ formed on the circumference of drum 5A inevitably has a large thickness when the both end portions of the laminate are overlapped and joined on the drum. Thus, even if the stitching roller is applied over joint PJ, air may remain at joint PJ, and if the remaining air expands by vulcanization molding of the green tire, joint PJ of laminate P may peel off.
In addition, since the end portions of laminate P form the joint at one location on the circumference of drum 5A in this technique, peeling-off of the joint of the inner liner of the molded green tire may cause a damage to an adjacent carcass ply.
It has been proposed in conventional techniques to use a thermoplastic elastomer for an inner liner with the intention to achieve weight saving of a pneumatic tire. However, the material, which is thinner and has a higher air permeation resistance than an inner liner made of butyl-based rubber, is inferior to the inner liner made of butyl-based rubber in vulcanization adhesive strength with insulation rubber and carcass ply rubber adjacent to the inner liner.
In particular, if the joint of the inner liner has a weak adhesive strength, the joint may peel off during driving, which may cause a reduction in the internal pressure of the tire, and burst of the tire. Further, since the joint has a structure in which another member is exposed inside, an air leakage path may be formed, and a reduction in the internal pressure of the tire is likely to be caused.
Japanese Patent Laying-Open No. 2009-208444 (PTD 1) discloses a technique of molding an unvulcanized tire by bonding an inner liner film and an unvulcanized rubber sheet with both ends in an extending direction being displaced from each other, and winding the tacky body on a drum.
However, in order to displace the both ends in the extending direction from each other, it is necessary to cut each member one by one to have a constant size, and individually bond the members with being displaced from each other, which may deteriorate productivity. Further, depending on the bonding method, accuracy is deteriorated and air remains between the films, which may cause a damage during vulcanization of the tire.
Japanese Patent Laying-Open No. 2007-291256 (PTD 2) discloses a pneumatic tire including a rubber composition for an inner liner containing an ethylene-vinyl alcohol copolymer in the range of 15 to 30 parts by mass relative to 100 parts by mass of a rubber component made of natural rubber and/or synthetic rubber. However, this technique is not preferable from the viewpoint of weight saving of tires since the inner liner has a large thickness of 1 mm.
Japanese Patent Laying-Open No. 9-165469 (PTD 3) discloses a nylon film used as an inner liner. The document discloses manufacturing a pneumatic tire by subjecting the nylon film to RFL treatment and thereafter bonding the nylon film to a tire inner surface or a carcass layer with a rubber cement made of a rubber composition.
This technique has a problem that it results in complicated steps. Further, in a vulcanization step in which vulcanization molding is generally performed with an unvulcanized tire accommodated within a mold being pressed against an inner surface of the mold from an inner side of the unvulcanized tire, the inner liner made of the nylon film sticks and adheres to a bladder and is damaged when the bladder is heated during vulcanization.
Japanese Patent Laying-Open No. 2010-013646 (PTD 4) proposes improving adhesive strength by using petroleum resin or terpene resin as a tackifier, for an SIBS as a thermoplastic elastomer. However, a polyamide-based polymer is blended in addition to the SIBS, causing a reduction in flex crack resistance.
Further, Japanese Patent Laying-Open No. 2010-100675 (PTD 5) proposes improving adhesiveness with carcass ply rubber by using natural rosin, terpene, chromane indene resin, petroleum resin, alkylphenol resin, or the like as a tackifier, for a blended material of an SIBS and a sulfur-crosslinkable polymer.
However, in the technique of blending 10 to 300 parts by weight of the sulfur-vulcanizable polymer relative to 100 parts by weight of the SIBS, when the sulfur-crosslinkable polymer is less than or equal to 100 parts by weight, the SIBS serves as a matrix (sea portion) and the sulfur-crosslinkable polymer serves as a domain structure (island portion), and adhesive strength with the carcass rubber at a contact interface is not improved. Further, when the sulfur-crosslinkable polymer is more than or equal to 100 parts by weight, gas barrier property is deteriorated in other than butyl rubber, and adhesive strength is deteriorated in butyl rubber. In addition, depending on a polymer to be blended, tackiness is increased, and it is not possible to fabricate a film with a thickness of 600 μm or less.
In International Publication No. 2008-029781 (PTD 6), a tire is manufactured using strips of a film laminate obtained by blending a thermoplastic resin with a thermoplastic elastomer. By employing a laminate, gas barrier property and adhesiveness can be improved, which enables junction between the ribbon-shaped strips. However, in this technique, an unvulcanized green cover of the film laminate has a constant gauge, and if the gauge is thinned, a vulcanized tire may have a thinned finish at a buttress part or the like.